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The Effect of Sea Surface Temperature on Hurricanes

This video segment adapted from NOVA scienceNOW highlights research that supports the idea that warmer oceans generate and sustain more intense hurricanes. Ongoing monitoring of sea surface temperature (SST) has supplied evidence that the world's oceans warmed 0.5°C between 1970 and 2005. Because hurricanes rely on warm water to release heat into the upper atmosphere and create spiraling winds, any additional energy can result in increased intensity. The video examines factors scientists use to predict hurricane behavior, and states that the complex nature of hurricane formation makes predicting with a high degree of accuracy very difficult.

This video is available in both English and Spanish audio, along with corresponding closed captions.

Until fairly recently, most scientists denied that global climate change and hurricane activity were related in any way. They said that hurricanes were subject to a separate cycle of oceanic and atmospheric activity that seemed to shift every 25 years or so. However, an increasing body of evidence now suggests that hurricane intensity—a measure of strength—is directly linked to rising global temperatures.

A hurricane begins when winds blowing across the surface of warm ocean water converge to form clusters of strong thunderstorms known as tropical disturbances. The heat of the water provides the energy these tropical storms need to build. As they do, water evaporates from the ocean surface and enters the air as water vapor. As the vapor rises and cools, it condenses and forms clouds, a process that releases heat. The heat warms the center, or eye, of the storm, which in turn causes more water to evaporate. Meteorologists call this cycle a "heat engine." As long as a storm has access to warm ocean water, this engine will continue to drive the storm's growth.

Since 1970, climate scientists have noted a gradual warming of the world's oceans. Most attribute this warming to an increase in greenhouse gas concentration in the atmosphere. In trying to determine whether there is a definitive link between rising sea surface temperature (SST) and hurricane intensity, scientists have noted that a 0.5ºC rise in global SST between 1970 and 2005 resulted in a 4 percent increase in atmospheric water vapor. Again, water vapor is associated with producing larger and more powerful hurricanes, so there may be a connection.

The Saffir–Simpson Hurricane Scale is used to classify hurricanes according to their intensity level. In order to be classified as a Category 1 hurricane, a tropical cyclone must have maximum sustained winds of at least 119 kilometers (74 miles) per hour. The highest classification on the scale, Category 5, is reserved for storms with winds exceeding 249 kilometers (155 miles) per hour. The classifications are intended primarily for use in measuring the potential damage and flooding a hurricane will cause upon landfall.

Scientists are under a lot of pressure to accurately predict hurricane direction and intensity. As Hurricane Katrina—a Category 4 storm that inundated New Orleans and nearby coastal areas in 2005—demonstrated, lives and property are at risk. While scientists have gotten pretty good at predicting the direction of a hurricane, they are not as good at predicting intensity. This is largely because they lack tools that enable them to read all the changes in terrain and water temperature that might cause a hurricane to strengthen or weaken. Using satellites equipped with weather radar, they are beginning to better visualize the internal structure of hurricanes. This may enable them to detect signs that a hurricane is about to enter an intensification phase, which can aid hurricane preparedness.

Here are suggested ways to engage students with this video and with activities related to this topic.

Beginning a lesson: Show photos of devastation caused by past hurricanes. You might start by going to the NOAA Photo Library and checking out the Weather Service section.

Viewing the video:Use the following suggestions to guide students’ viewing of the video.

Before: Using a map of the world, have students put sticky notes on areas where they think hurricanes form. Ask, Why do you think hurricanes form in that place/those places? Then have them predict a hurricane path based on the point or points of origin they selected. Ask, Why do you think hurricanes would follow this path/these paths? After you have elicited some of their ideas, you may use the map to show where hurricanes form and the general paths they take. (Note: Three names describe the same basic weather phenomenon. Hurricanes typically form in the Atlantic Ocean, typhoons in the North Pacific, and tropical cyclones in the Indian Ocean and South Pacific.)

During: Ask students to jot down the different variables mentioned in the video that factor into hurricane formation and intensity. A version of the graph of Storm Intensity and Sea Surface Temperatures referenced in the video is available to show to students or to print out.

After: Focus on other environmental factors that are being impacted by warming seas. Here are some questions you might ask:

If ocean temperatures are rising, what do you think the effect will be on ice at the poles? [reduced area of sea ice cover, thinner cover]

How do you think warming waters are affecting marine organisms? [forcing some species to leave their normal habitats in search of colder waters]

If some marine organisms are forced to leave their normal ocean habitats, how might this affect entire ecosystems? [if an important food source disappears, populations further up the food chain may suffer; likewise, if a top predator is forced to leave, the populations of those animals below them on the food chain may grow out of balance]

Doing research projects—individual: Have students ever experienced a hurricane, nor’easter, blizzard, tornado, or other intense weather phenomenon where they live? If so, ask them to tell about it. They should include details about their anticipation of and preparation for the storm’s arrival as well as any damage it might have caused. They might also interview parents or other relatives about their experiences. Have students report back to the class.

Doing research projects—individual: Have students research what their local government suggests residents do in the event of a hurricane. (If your region does not have hurricanes, have students ask about weather events that do apply to your community.) Tell students to write down evacuation procedures and plot out their evacuation route. Have them report back to the class.

Doing research projects—individual or group: Hurricanes gain strength when they're over open water, and lose strength when they reach a coastline. Have students do some research on their own at home or in small groups in the classroom to track the progress of Hurricane Katrina, Hurricane Wilma, or some other recent Atlantic hurricane that made landfall. How did wind speeds vary along the storm’s path? What other factors may have influenced the storm's intensity? Why did the storm eventually lose hurricane status?

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